Recently, there has been proposed to incorporate a filter for capture of particulates in exhaust in an engine exhaust system and to arrange a selective reduction catalyst capable of selectively reacting NOx with ammonia even in the presence of oxygen in the same engine exhaust system and downstream of the filter, urea water for reduction being added between the filter and the selective reduction catalyst so as to capture the particulates and reduce the NOx.
When such a way is to be employed, a distance from the added position of the urea water to the selective reduction catalyst must be prolonged to sufficiently ensure time for thermal decomposition of the urea water added to the exhaust into ammonia and carbon dioxide. However, such axially aligned arrangement of the filter and the selective reduction catalyst substantially impairs mountability on a vehicle.
Then, the applicant has already proposed in Japanese patent application No. 2007-29923 an exhaust emission control device as shown in FIGS. 1 and 2 which has excellent mountability on a vehicle and which can prolong a distance from added position of urea water to selective reduction catalyst.
In the proposed exhaust emission control device, a casing 7 encasing a filter 5 for capture of particulates and a casing 8 encasing a selective reduction catalyst 6 for reduction of NOx are arranged side by side in such a manner that exhaust 3 has the same flow direction in the respective casings, an exhaust inlet end of the casing 7 being connected to an exhaust pipe 4 contiguous with an exhaust manifold 2 of a diesel engine 1, an exhaust outlet end of the casing 7 being connected to an exhaust inlet end of the casing 8 through a communication passage 9.
The casing 7 encasing the filter 5 extends in a fore-and-aft direction of a vehicle (truck) and is arranged closely adjacent to a side of a side member in a frame; the casing 8 encasing the selective reduction catalyst 6 extends in the fore-and-aft direction of the vehicle and is arranged closely adjacent to a bottom of a bed or rear deck so as to be positioned laterally external of the casing 7 in the vehicle.
The communication passage 9 has a collection chamber 9A encircling the exhaust outlet end of the casing 7 to upwardly collect the exhaust 3 having passed through the filter 5, a mixing pipe 9B which guides the exhaust 3 having passed through the chamber 9A to a vicinity of an exhaust inlet end of the casing 8 in a direction reverse to that of the flow in the casing 7 and a dispersion chamber 9C which encircles the exhaust inlet end of the casing 8 to guide the exhaust 3 having passed through the mixing pipe 9B in laterally dispersed manner into the selective reduction catalyst 6 in the casing 8.
Arranged at the mixing pipe 9B and adjacent to the collection chamber 9A is urea water addition means 10 for addition of urea water to the exhaust 3.
Arranged upstream in the casing 7 is oxidation catalyst 11 for treatment of unburned fuel in the exhaust 3. Arranged downstream in the casing 8 is ammonia reducing catalyst 12 for oxidization treatment of surplus ammonia.
In the exhaust emission control device, the casings 7 and 8 within each of which the exhaust 3 flows in the fore-and-aft direction of the vehicle are arranged side by side laterally of the vehicle and the mixing pipe 9B is positioned above the casing 8 so as to prevent the overall device from becoming oversized both in the fore-and-aft direction and laterally of the vehicle, which makes it possible to improve the mountability on the vehicle, prolong the distance from the added position of the urea water to the selective reduction catalyst and substantially ensure time for decomposition of the urea water added to the exhaust 3 into ammonia and carbon dioxide.
An exhaust emission control device with a filter for capture of particulates and a selective reduction catalyst arranged side by side has been proposed for example in the following Patent Literature 1.    [Patent Literature 1] JP 2005-155404A